There are a number of time delay generators known in the prior art, including those employing Gilbert cell circuit configurations. In each of the previous time delay generators, the mixing of delayed and undelayed input signals is regulated by an analog control signal coupled to the differential transistor pairs of the mixing circuit. The application of a single analog control signal causes two significant problems substantially overcome by the present invention. First, the time delay response in the prior art Gilbert cell time delay generators is non-linear, often requiring a pre-distortion stage to compensate and correct the analog control signal. With the time delay of the output signal charted on the vertical axis of FIG. 1, the time delay response 100 of the prior art time delay generator varies non-linearly across the entire analog control signal voltage range charted on the horizontal axis. Second, external noise on the analog control signal introduces phase noise in prior art time delay generators. The degradation in phase resolution is significant and is commonly the limiting factor in the performance of high frequency time delay generators operating in and above the microwave band.
Therefore, it would be desirable to have means, first, to control the mixing of delayed and undelayed input signals in a manner that is linearly responsive to one or more control signals and, second, to prevent control signal noise from propagating into the mixer where it causes phase noise and phase resolution degradation.